1. Field of the Invention
The present invention relates to a wireless ATM access system, particularly to a cell flow synchronization establishing system between BS and MS for resolving state incoincidence caused between MS and BS.
2. Description of the Related Art
Generally, in a wireless ATM access system for carrying out communication between BS (Base Station) and MS (Mobile Station, mobile terminal) by using ATM (Asynchronous Transfer Mode: asynchronous communication system) cell, when abnormality in communication is caused by arbitrary communication connection, in order to resolve incoincidence of a communication state between MS and BS, there is carried out a processing of establishing a transmission and reception order of cell stream, that is, a processing of establishing synchronization of cell flow (initialization).
In a conventional cell flow synchronization establishing system of wireless ATM access system, when communication abnormality is detected in arbitrary communication connection, automatically or in accordance with control terminal operation by a maintenance operator, an instruction for establishing synchronization of cell flow is outputted individually to corresponding BS and MS.
FIG. 5 is an explanatory view showing a conventional wireless ATM access system, FIG. 6 is a sequence diagram showing a conventional procedure of establishing synchronization of cell flow and FIG. 7 is a block diagram showing a cell flow synchronization establishing system.
In FIG. 5, an ATM switching station (hereinafter, referred to as MSC, Mobile Switching Center) 1 contains communication connections from a fixed ATM network via NNI (Network Node Interface), contains communication connections with BS 2A and BS 2B via UNI (User Node Interface) and mutually switches and connects the respective communication connections based on control a switch control unit (SW-CNT).
Further, BS 2A or 2B contains MS3 via a wireless I/F (interface).
There are installed cell store buffers 20B and 40B for temporarily storing cell transmitted and received via the wireless section and buffer units 20 and 40 for carrying out communication control in the wireless section by controlling the cell store buffers 20B and 40B at BS 2A, BS 2B and the wireless I/F side of MS3.
As shown by FIG. 7, constitutions of the buffer units 20 and 40 are substantially the same as each other.
A sequence number control buffer 23 is a buffer for controlling a sequence number for guaranteeing reproduction of cell stream, that is, orderness of stream of normal reception cell and reception cell retransmitted by ARQ (Automatic Repeat Request).
A CDV (Cell Delay Variation) control buffer 22 is a buffer for controlling timing for executing reproduction of cell stream by ARQ.
A cell transmission and reception pointer control buffer 24 is a buffer for controlling a pointer value of cell transmitted and received by the cell store buffer 20B.
Further, a high layer interface unit 21 initializes the sequence number control buffer 23, the CDV control buffer 22 and the cell transmission and reception pointer control buffer 24 by setting I/O register in accordance with an instruction of establishing synchronization from a high layer 5.
As shown by FIG. 6, when synchronization establishment instructions 300 and 301 are outputted from the high layer 5 to the buffer units 20 of BS 2A and BS 2B, after making ON flags indicating that the synchronization establishment is being processed (steps 321, 341), corresponding I/O registers are set by which the sequence number control buffer 23, the CDV control buffer 22 and the cell transmission and reception pointer control buffer 24 are initialized (steps 322, 342).
At this occasion, pointer values held by the respective control buffers are initialized to zero and in accordance with completion of initialization (steps 323, 343) the flags of the high layer interface units 21 are set to OFF.
After outputting the synchronization establishment instructions 300 and 301, the high layer 5 successively checks flags of the respective buffer units 20 and 40 at steps of buffer state monitoring 302, 303.
Further, the synchronization establishment instructions 300 and 301 are repeatedly outputted until flag OFF is confirmed, completion of synchronization establishment is confirmed in accordance with flag OFF, communication control states coincide with each other between BS 2A and MS3 and it is determined that the transmission and reception order of cell stream (cell flow), that is, synchronization of cell flow is established.
However, according to such a conventional cell flow synchronization establishing system, no consideration is given to handoff in accordance with movement of MS to outside of a service area and there poses a problem in which when handoff is caused and a processing of switching communication connection is carried out in the midst of clearing a sequence number of a cell in which synchronization of cell flow is being processed and a buffer of BS controlling transmission and reception pointer, a processing of clearing a buffer of old BS before switching the communication connection is interrupted.
Normally, a control pointer of the transmission and reception cell buffer is controlled by the sequence number of a cell and reading and writing of the transmission and reception cell buffer is carried out in reference to the control pointer of the transmission and reception cell buffer.
Accordingly, the processing of clearing the control pointer of the transmission and reception cell buffer (clearing pointer value to “0”) signifies clearing of the transmission and reception cell buffer, that is, abandoning a cell stored in the transmission and reception cell buffer.
Therefore, when handoff is caused, communication connection in respect of BS executing the buffer clearing processing is cut with the handoff as a boundary, monitoring from the high layer cannot be carried out and the buffer clearing processing of old BS before switching the communication connection is interrupted in the midway of processing.
Further, depending on timing of occurrence of handoff, the buffer clear processing is instructed from the high layer to new BS after switching the communication connection after occurrence of the handoff to thereby cause to start an incomplete buffer clear processing.
That is, with the handoff as a boundary, the buffer clear processing is carried out in respect of old BS before switching the communication connection and in respect of new BS after switching the communication connection.
Thereby, as a result, not only cell loss is caused but also cell stream cannot be guaranteed on the side of a reception station (indicating MS in the case of a descending circuit, BS in the case of an ascending circuit).